Display devices, including visual display devices, plan an important part in modern technology and commercial devices available to the public. Typical display devices are cathode ray tubes for use in television sets, tubes used in monitoring devices and tubes used in projection televisions. Also of commercial importance are X-ray imaging devices in which typically X-ray radiation is converted into visible radiation. Phosphors are used to convert various kinds of energy, particularly electromagnetic radiation energy and electron beam energy into radiation in the visible region or radiation region directly adjacent to the visible region such as the infrared region or ultraviolet region.
Cathode ray tubes (CRTs) are especially useful as display devices. They are extensively used in direct view and projection television sets, monitors for computer terminals, television and avionics systems, etc. In many applications (such as projection tubes), high image brightness is required which can only be obtained by the use of a very high power density electron beam. Such high power densities often degrade conventional cathode ray tubes and therefore limit the lifetime of high intensity cathode ray tubes. Even with conventional television tubes extensively used by the public, degradition of the phosphor screen is often apparent.
A significant advance in the development of high intensity cathode ray tubes was the discovery that certain luminescent epitaxial garnet films on single crystal substrates could withstand much higher power densities than with powder phosphors without tube degradation (see, for example, J. M. Robertson et al, Applied Physics Letters, 37 (5), pp. 471-472, Sept. 1, 1980). several systems were examined using yttrium aluminum garnet in the epitaxial layers. The activators examined were Tb, Eu, Pr, Tm and Ce. The epitaxial layers were grown by liquid phase epitaxy using a PbO-B.sub.2 O.sub.3 flux.
Although there are a number of ways to produce these luminescent layers such as chemical vapor deposition, liquid phase epitaxy and sputtering, all are usually expensive and do not adapt well to mass production. It is highly desirable to have a procedure for producing luminescent display devices which are highly efficient, have high resolution and can be mass produced at low cost. Particularly attractive is a procedure for making cathode ray tubes at a cost comparable with presently mass produced cathode ray tube with improved performance including absence or reduced degradation of the phosphor, higher resolution and higher brightness.
The sol-gel procedure is a well-known procedure for making glass and crystalline solids. It is also suitable for the deposition of thin amorphous and crystalline films. The methods of deposition of oxide layers from organic solutions are reviewed by H. Schroeder, "Oxide Layers Deposited from Organic Solutions,"pp. 87-141 in Physics of Thin Films 5, edited by G. Hess and R. E. Thun, Academic Press, New York, 1969. A number of other references may also be useful in understanding the invention including H. Dislich, "Glassy and Crystalline Systems from Gels: Chemical Basis and Technical Application," Journal Non-Crystalline Solids, 57, 371-88 (1983); H. Dislich, "Glassy and Crystalline Systems from Gels, Chemical Basis and Technical Application," Journal Non-Crystalline Solids 63, 237-41 (1984); P. Hinz and H. Dislich, "Anit-Reflecting Light-Scattering Coatings via the Sol-Gel Procedure,"ibid., 82, 411-16 (1986); S. Sakka, K. Kamiya K. Makita, and Y. Yamamoto, "Formation of Sheets and Coating Films from Alkoxide Solutions, " ibid., 63, 223-35 (1984).
Several references describe the of luminescent materials in conjunction with gel processes (e.g., U.S. Pat. No. 3,927,224 issued to Leon Levene on Dec. 16, 1975 and U.S. Pat. No. 3,816,328 issued to Leon Levene on Jun. 11, 1974). These references describe the use of sol gels for incorporation of luminescent material.